This invention relates to the production of image information of objects containing nuclear spin magnetic moments whose spatial distribution of density or relaxation time is detected by magnetic resonance techniques.
NMR imaging techniques require generally the application to an object of a static magnetic field, radio frequency excitation of nuclear spins and the application of magnetic field gradients along orthogonal axes. One of the principal problems in achieving a satisfactory image has, using known methods of measurement, been the production of a highly homogeneous static magnetic field. Poor field homogeneity results in a reduction in the signal to noise ratio in the image due to the requirement for large compensatory signal bandwidths. In addition various phenomena arising from an inhomogeneous field may further degrade the image quality.
It is known, for example from Edelstein et al, Phys. Med. Biol. Volume 25 (1980) pages 751 to 756 to provide a gradient and radio frequency pulse sequence which comprises a selective (90.degree.) excitation of a thin slab of spins perpendicular to a first axis, projection of the spin density on to a second axis orthogonal to the first and phase encoding of spins on a third axis orthogonal to the first and second axes to allow spatial discrimination along the third axis, the phase encoding being achieved by the application of a half-sine wave pulse which is applied for the same time in each pulse sequence but having a strength varied in order to give spins at different "heights" varying amounts of phase twist according to the varying strength of the half-sine wave gradient pulses in the said third direction. A set of signals, representing values of the free induction decay signals measured at corresponding times and associated with different amplitudes of the gradient pulses in the third axis provide a two dimensional data set which when Fourier transformed in two dimensions provides image information in the selected plane. The technique essentially employs the precession frequency to encode spatial information in one direction and the phase of the precessing spins to encode in another direction. The technique provides satisfactory results if the static field is homogeneous. However, if the field is not homogeneous, that is to say the inhomogeneity is significant compared with the applied gradients, the inhomogeneity causes a geometric distortion in the direction of the static gradient. It is possible to reduce the geometric distortion by increasing the strength of the static gradient. However, this requires an increased signal bandwidth which will result in a reduction in the signal to noise ratio. Alternatively, the static field strength may be reduced but this is again undesirable because the signal to noise ratio decreases as the static field strength decreases.
It is further known, for example from GB-2 124 388 A and EP 0 109 633 A, that the use, in NMR imaging, of phase encoding in two directions at right angles in the same plane will allow spatial discrimination of spin densities in the said two directions. The use of phase encoding in two directions removes the geometric dependence of the image on the static field and accordingly could permit the performance of NMR imaging wherein the static magnetic field is significantly homogeneous. However, an unacceptable time penalty is paid for this method. Whereas the method of Edelstein takes N units of time to produce an N x N image, the two dimensional phase encoding technique requires Nz units of time for data collection. Imaging times of the order of hours are required to produce images of an acceptable spatial resolution. This is obviously not feasible where human subjects are involved.